1. Field of the Invention
First, the present invention relates to an electrophotographic photoconductor that can realize excellent service durability, stable electrical characteristics, and high quality image formation for a long period, using a photoconductive layer with high abrasive resistance, good film surface properties, and good electrical characteristics.
Second, the present invention relates to an electrophotographic photoconductor that can realize excellent service durability, stable electrical characteristics, and high quality image formation for a long period, using a surface layer with high abrasive resistance, a smooth film surface, and little variation in electrical characteristics dependent on the environment.
Further, the present invention relates to an image formation method, an image formation apparatus and a process cartridge for image formation apparatus using the aforementioned high quality photoconductor with a long operating life.
2. Description of the Related Art
Recently, an organic photoconductor (OPC) is frequently used in a copying machine, a facsimile machine, a laser printer, and a complex machine thereof, due to good performance and various advantages of it, instead of an inorganic photoconductor. As the reasons, for example, (1) excellent optical properties such as a wider wavelength range for light absorption and higher rate of absorption, (2) excellent electrical characteristics such as high sensitive and stable charging property, (3) a wide scope of material selection, (4) easier manufacturing, (5) lower cost, and (6) no toxicity can be listed.
On the other hand, recently, the achievement of high durability of a photoconductor has been desired for the miniaturization of a photoconductor promoted in accordance with the miniaturization of an image formation apparatus, the speeding up of a machine, and the tendency of maintenance-free.
From this viewpoint, since a surface layer of the organic photoconductor is based on a low-molecular-weight charge transportation material and an inactive polymer, the organic photoconductor is generally soft, and, therefore, has a disadvantage of easily causing abrasion by mechanical load from a development system or a cleaning system, when the organic photoconductor is used repeatedly in an electrophotographic process.
In addition, with the miniaturization of the particle diameters of toner particles for the requirement of achieving a high quality image, the increase of the rubber hardness and the contact pressure of a cleaning blade has to be made for improving a cleaning property, which increase is a factor of accelerating the abrasion of the photoconductor. Such abrasion of the photoconductor lowers the sensitivity and degrades electric characteristics such as the charging property, so as to cause the lowering in image density and improper imaging such as background contamination. Also, the damage caused by local abrasion results in insufficient cleaning, and therefore, an image with linear contamination. In the present circumstances, the operating life of the photoconductor, that is, the replacement of the photoconductor, is regulated by the abrasion and the damage.
Accordingly, it is necessary to reduce the aforementioned abrasion for achieving the high durability of an organic photoconductor, and further, an organic photoconductor having good surface properties are required for giving an excellent cleaning property and a transcription property to the organic photoconductor. These problems are required to be solved in the art.
As techniques for improving abrasive resistance of a photoconductive layer, (1) the use of a curable binder in a surface layer (ex. see Japanese Laid-Open Patent Application No. 56-48637), (2) the use of a polymeric charge transportation material (ex. see Japanese Laid-Open Patent Application No. 64-1728), (3) dispersing an inorganic filler in a surface layer (ex. see Japanese Laid-Open Patent Application No. 4-281461) can be provided.
Among these techniques, (1) the use of a curable binder tends to elevate a residual potential and cause lowering in image density due to a low compatibility with a charge transportation material and impurities such as a polymerization initiator and an unreacted residue. Also, (2) the use of a polymeric charge transportation material and (3) dispersing an inorganic filler can improve abrasive resistance to some extent, but have not satisfied sufficiently the resistance required for an organic photoconductor. Additionally, (3) dispersing an inorganic filler tends to elevate a residual potential and cause lowering in image density due to a trap existing on the surface of the inorganic filler. Moreover, these techniques denoted by (1), (2), and (3) have not satisfied sufficiently the overall durability that includes electric durability and mechanical durability required for an organic photoconductor.
Furthermore, a photoconductor that contains a material obtained by curing multi-functional acrylate monomers for improving the abrasive resistance and the damage resistance is known (see Japanese Patent No. 3262488). With respect to the photoconductor, the patent discloses that the material obtained by curing multi-functional acrylate monomers is contained in a protective layer provided on a photoconductive layer. Also, the patent discloses that the protective layer may contain a charge transportation material but no specific explanation. Further, when a low-molecular-weight charge transportation material is simply contained in a surface layer, there is a problem of the compatibility with the cured material, whereby the precipitation of the low-molecular-weight charge transportation material and the production of a crack can be caused to lower the mechanical strength.
Although the patent also discloses that a polycarbonate resin is contained for improving the compatibility, the content of the acryl monomers to be cured is reduced and, consequently, sufficient abrasive resistance cannot be achieved. Additionally, with respect to a photoconductor that contains no charge transportation material in the surface layer, the patent discloses that the surface layer is made be a thin film against the lowering in the electric potential of a light-exposed portion. However, since the film thickness is small, the operating life of the photoconductor is short. Further, the environmental stability of charging electric potential and the electric potential of a light-exposed portion is low and the values of them varies widely dependent on the environment factors such as temperature and humidity and cannot be kept at a sufficient values at present.
Instead, as a technique for improving an abrasive resistance of a photoconductive layer, it is known that a charge transportation layer formed from a coating liquid that contains a monomer having a carbon-carbon double bond, a charge transportation material having a carbon-carbon double bond and a binder resin is provided (ex. see Japanese Patent No. 3194392). The binder resin includes that of having a carbon-carbon double bond and reactivity to the charge transportation material and that of having no carbon-carbon double bond and no reactivity to the charge transportation material.
The photoconductor attracts attention since the photoconductor has both abrasive resistance and good electrical characteristics, but when a binder resin having no reactivity is used, the compatibility of the binder resin with a cured material produced by the reaction of the aforementioned monomers and the charge transportation material is low and layer separation or the production of surface irregularity are made at the time of cross-linking. As the result, it is observed that the photoconductor tends to cause improper cleaning.
As described above, the binder resin disturbs the curing of the monomers. Further, the patent discloses a two-functional monomer as the monomer used in the photoconductor but sufficient cross-link density cannot be obtained by the two-functional monomers since the number of functional groups of the monomer is small and the photoconductor does not satisfy the sufficient abrasive resistance.
Also, when the binder resin having reactivity, due to the small number of functional groups contained in the monomer and the binder resin, it is difficult to balance the extent of monomer coupling in the charge transportation material and the cross-link density of the charge transportation material, and the electrical characteristics and the abrasive resistance are insufficient.
Moreover, a photoconductive layer that contains a compound obtained by curing hole transportation compounds having more than one chain-polymerizable functional group in the molecule thereof is known (ex. see Japanese Laid-Open Patent Application No. 2000-66425).
However, since the bulky hole transportation compound in the photoconductive layer has more than one chain-polymerizable functional group, distortion is caused and the internal stress increases in the cured compound. As the result, the surface layer may easily become rough or produce a crack with time, so that the surface layer does not have sufficient durability.
Thus, the photoconductor having a cross-linked photoconductive layer obtained by chemically bonding the charge transporting structures in these conventional techniques does not have the sufficient overall characteristics at present.